Open cell polymeric foams are very important in everyday life as good sound and thermal insulators. Their mechanical properties as shock absorbing materials are also conveniently exploited in padding and stuffing. Polymer foams have a distinctly different behaviour in tension and compression, displaying a certain rigidity in tension but being very compliant in compression. The reason for this behaviour is to be looked for in the microstructure: in compression the cell walls and struts composing the microstructure buckle, thus the material is able to deform at a very low, almost constant applied external stress. The modelling of this phenomenon can be undertaken using Cellular Materials Mechanics (Gibson & Ashby, 1997), reaching a good number of interesting conclusions and matching the experimental results to a certain extent. In this work it is proposed to use a Continuum Mechanics approach to the problem. This can result in an easier implementation of the model into numerical codes and, at the same time, the model can be freed from assumptions regarding a very regularly ordered microstructure. Analysing the results of Papka and Kyriakides (1994), it seems clear that buckling does not occur everywhere at the same time in the material, but rather in a mixture-like way, thus Multinetwork Theory (Wineman & Rajagopal, 1990, Rajagopal & Wineman, 1992) seems an ideal setting for this problem. For the activation criterion, the approach that is used is very similar to the one of Rao & Rajagopal (2000 and 2001), who studied strain induced crystallization in polymers.
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